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ATU fun

K

KRXOEngineer

I'm having a bit of "fun" with an old ATU at a 500 watt bean-warmer AM. The have a pair of caps in series that is taped in the center of them with a coil to ground. The mica caps that are in the ATU are both .001 and the coil is a Gates 87-fa-4634 using 8 turns of the coil. I'm reading about a half a amp low on the station even though the Gates One transmitter is reading 500 watts. I'm inclined to think the matcher on the transmitter is adjusted to make the transmitter happy, so what the direct power reading on the front of the transmitter is saying isn't reality. I've taken a OIB-1 and measured at the transmitter output into the coax. It seems to be x=13.5 R=45. Out at the ATU input the OIB reads X=0 R=37. The tower itself is licensed at 31.7 at 3.97. It's reading about 3.5amps. Another slight oddity it seems the tower itself measures 30.5.

So this leaves me wondering if a capacitor been replaced over the years with the wrong value, throwing stuff off. What value should it really be? Does anyone have any info on what the Gates coil's value is (full length or otherwise)?

Thanks!
 
As the tower base resistance goes down, the antenna current needs to be greater to maintain desired power. P=I2R. Therefore you may be running less power than you think.

With a single adjustable component in the ATU, it's going to be impossible to match the tower to the transmission line without changing out capacitors for different values. A more common configuration is a series L (inductor), or one that is in series with the input and a shunt cap to ground with a center tap for the output leg. The output leg would have a cap to match the inductive reactance. An added advantage is that this functions as a low pass filter (knocking down harmonics), whereas the series cap configuration is a high pass filter.

You obviously do not have a match to the transmission line (unless you have 37 ohm transmission line ;D), so the transmission line is acting as a transformer giving you another odd value as the transmitter output. With that mismatch, you're probably losing power in the transmission line. I'd ask if there were any hot spots, but with only 500 watts, you probably wouldn't notice. Out of curiosity, what is the line and how long is it?
 
I think it's 1 5/8 foam and about 100 feet long. That makes sense that it's acting as a weird transformer, etc. ;D.

Thanks again for the help!
 
KRXOEngineer said:
I think it's 1 5/8 foam and about 100 feet long. That makes sense that it's acting as a weird transformer, etc. ;D.

Thanks again for the help!
Click to expand...
Who uses 1 5/8" line for a 500 watt AM?
 
A poorly engineered one. I'm not a 100 percent certain on the coax size to be honest. I'll double check it. This station has be mis-engineered for years before I got unlucky and received a phone call from them years ago. Many things have been replaced over time including the lousy transmitter and console they had, over the years, but the ATU isn't one of them. Hopefully I can get them to do the "right thing" and just replace it. We shall see.
 
KRXOEngineer said:
A poorly engineered one. I'm not a 100 percent certain on the coax size to be honest. I'll double check it. This station has be mis-engineered for years before I got unlucky and received a phone call from them years ago. Many things have been replaced over time including the lousy transmitter and console they had, over the years, but the ATU isn't one of them. Hopefully I can get them to do the "right thing" and just replace it. We shall see.
Click to expand...
Actually, there's no harm in using 1 5/8" transmission line for a 500 watt AM station, unless you consider the damage to your budget. 1/2" line is more than sufficient at a small fraction of the cost. Using 1 5/8" line for 500 watts is similar to building a 6 lane expressway so grandma can drive a mile to the grocery store. It will work fine, but what a waste of money!

If you look at the line, it will probably have some numbers on it that can tell you what size it is. A Google search of the number will often reveal the size of line. I know precious little about AM matching networks, but there are any number of engineers on here that are well versed on the topic.
 
KRXOEngineer said:
A poorly engineered one. I'm not a 100 percent certain on the coax size to be honest. I'll double check it. This station has be mis-engineered for years before I got unlucky and received a phone call from them years ago. Many things have been replaced over time including the lousy transmitter and console they had, over the years, but the ATU isn't one of them. Hopefully I can get them to do the "right thing" and just replace it. We shall see.
Click to expand...
Maybe the owner had a dream of adding a couple of zero's to his 500 watt flamethrower down the road ;D
 
Who uses 1 5/8th? Someone who had a hundred feet of it in good condition oin a spool. Occam's Razor applies. And a variation in the tower of 3% or so isn't earthshaking, but I'd sneak out one night in the wet and one in the dry and see how much it changes... you may have an expired ground system as well. If the resistance is fairly stable, set the ATU back to the original values and see how closely it matches. And go from there.
 
I'm sure that's exactly what happened. They must have got a deal on the coax many years ago and that's what's there now. I should check into the wet vs. the dry thing. There might be some variation for sure on what the tower impedance is. I'm checking with some ATU manufactures for the cost of the correct parts to rebuild the ATU. I think it's pretty much a given it's far from right, based on what readings I've been able to come up with so far. After I really get 50 ohms 0J at the input to the ATU I think I'll file for direct measurement at the transmitter output. That way if the tower impedance changes a bit I won't have to go through the legal hell again at least. Then I can try to figure out why it might be changing, if it does.

Does anyone have ideas on what values I aught to use for the ATU to give the proper match and bandwidth, etc? I've seen severl online calculations, but I'm at a bit of a loss on what's best configuration-wise and the appropriate values that go with it, etc.
 
Your existing ATU network is a "Tee". While this network can be made to work, you will need to add a variable capacitor on the shunt leg and you should use two coils rather than one. This will give you better control of the input/output impedance transformation and a wider range of control.
I would prefer a combination "Tee" and "Pi" network to give you the best control of impedance and bandwidth.
The advantage of using larger than required transmission line is that you get slightly less line loss.
At a transmission line length of only 100', the difference is minimal.
 
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